1. Field of the Invention
The present invention relates to a display control apparatus suitable for use for a display device using a ferroelectric liquid crystal.
2. Related Background Art
A CRT (cathode ray tube) has been used as a display device for a personal computer (hereinafter referred to as PC) and a work station (hereinafter referred to as WS). However, recently, a liquid crystal display device of a TN (twisted nematic) structure or an STN (super twisted nematic) structure has been used in a lap-top PC because of its superiority in light weight and thinness. Further, an FLC (ferroelectric liquid crystal) has many features desirable in a display element such as a bistable memory characteristic and high response. A display device which uses such a liquid crystal attains a high resolution by the memory characteristic of the element and it has been attracting attention as a liquid display device capable of displaying in a large screen because of a simple electrode process of a matrix structure.
In order to take synchronization in supplying image data from a display control apparatus to the display device, an external synchronization method or an internal synchronization method is used depending on a characteristic of the display device. In a liquid crystal display which uses liquid crystal which responds to an effective value of an applied voltage, synchronization means for a CRT signal is provided in a display controller to share an interface of the CRT with the display controller to permit the display by the external synchronization method. A display device having a non-linear active device provided for each pixel to synchronize with the CRT signal has also been known although it has experienced difficulties in the manufacturing process.
On the other hand, a liquid crystal display device which receives image data by the internal synchronization in order to use the property of a material to a maximum extent has been known. For example, in a ferroelectric liquid crystal, a displayable horizontal scan time varies with a temperature as shown in FIG. 3. Because of such a temperature characteristic of the material, in the display device which uses the ferroelectric liquid crystal, a display temperature of glass plates which hold the liquid crystal therebetween is detected and a period of reception of the image data is controlled by the internal synchronization. The liquid crystal display device has means for setting a horizontal display timing or a liquid crystal drive voltage by using an external temperature as a parameter. A horizontal display timing for the liquid crystal response speed is supplied to the display control apparatus as a reception synchronization signal of the image data.
In demonstration of a reliability evaluation test of the display device, it may be required to simultaneously display on a plurality of display devices. In such a case, if the CRT display device receives the image data by the external synchronization, the display is attained by arranging a distributor which causes the matching of the input impedance of the display device between the display device and the display control apparatus. In the liquid crystal display device which receives the image data by the external synchronization, the display is attained by arranging a distributor having a current buffer between the display device and the display control apparatus. This distribution method permits the activation of a plurality of display devices by using one host computer having the display device.
However, in the liquid crystal display device which displays by the internal synchronization, the same number of host computers 10 to 15 as that of display devices 55 to 60 are used in the prior art system as shown in FIG. 7. In the demonstration of the reliability evaluation test of the display device, the environment parameters of the display device are stable in many cases but some variation of the environment parameters is unavoidable. Accordingly, it is difficult to drive a plurality of display devices by one host computer because of the difference in the synchronization period of the display device which varies with different environmental parameters.
FIG. 4 shows a prior art display system for displaying on one ferroelectric liquid crystal display device 20 by one host computer 30. In a flow of image data, a work station 40 first writes update image data to a video memory provided in a display control apparatus 32 upon request of draw primitive. Then, the display control apparatus 32 is supplied with a horizontal synchronization signal shown in FIG. 5 from a display controller 21 built in the display device 20, and supplies image data d (a scan address e and image data f) to the display controller 21 at a timing shown in FIG. 5 in accordance with an image data transfer clock c. A horizontal scan period h of the horizontal synchronization signal a is determined by a temperature sensor 22 mounted on the glass plates which hold the liquid crystal element therebetween. The scan line address e and the image data f are discriminated by an address/data identification signal. A difference between the image data and the horizontal scan period h is adjusted by a period g.
The image data f transferred to the display controller 21 is divided into odd and even pixel data which are supplied to segment drivers 23 and 24. Then, the segment drivers 23 and 24 apply appropriate voltages to segment electrodes of matrix structure electrodes of a liquid crystal panel 26. This voltage causes a potential difference from a voltage applied to a common electrode from a common driver 25 to drive the liquid crystal element.
In this manner, the display by the display device 20 which displays by internal synchronization is attained. Accordingly, for the simultaneous display by the plurality of display devices, a plurality of display control apparatus 32 for synchronously supplying image data and a plurality of work stations 40 are used for the horizontal synchronization signals of different periods.
Since the ferroelectric liquid crystal has the property of the response time to the temperature as shown in FIG. 3, a method has been known in which the horizontal display timing corresponding to the response time of the liquid crystal is supplied to the display control apparatus using the temperature as a parameter, as a synchronization signal of the image data transfer. In this display method, the synchronization period for the image data transfer from the display device to the display control apparatus is controlled in accordance with the environmental temperature. In this case, if the supply period for one scan of image data supplied from the display control apparatus to the display device is longer than the period of the synchronization signal supplied from the display device, a loss of the image data occurs in the received data and a predetermined image is not displayed. On the other hand, if the supply period of one scan of image data supplied from the display control apparatus to the display device is shorter than the period of the synchronization signal supplied from the display device, a drive voltage in a period other than the draw period on the liquid crystal may be set to a common voltage to improve a contrast to some extent. Accordingly, the supply period of one scan of image data supplied from the display control apparatus to the display device must be shorter than the period of the synchronization signal supplied from the display device.